1. Field of the invention
The present invention relates to a semiconductor device and a method for manufacturing the same, and more specifically to a semiconductor device having a metal wiring layer and a method for manufacturing the same.
2. Description of related art
First, one example of the prior art will be described. As a technology for filling a connection hole such as a fine contact hole or a fine via-hole for interconnecting wiring conductors of different levels, a blanket tungsten chemical vapor deposition (blanket WCVD) has been reduced into practice, which is excellent in step coverage and low in resistance. In this blanket WCVD method, after a blanket tungsten film is formed, the blanket tungsten film is etched back by a dry etching so that the tungsten film of only a plug is caused to remain, and then, a film of a metal such as aluminum is formed thereon, and patterned to form a patterned wiring conductor.
Recently, a method has been proposed in which a wiring conductor itself is formed of the tungsten film, in order to omit the step of etching back the tungsten film. Now, this method will be described with reference to FIGS. 1A to 1D.
First, as shown in FIG. 1A, a silicon oxide film 2 is formed on silicon substrate 1, and a contact hole is formed in the silicon oxide film 2. An adhesion layer 3 is deposited in the contact hole by a sputtering. The adhesion layer 3 is suitable to be formed of a double layer of composed of a titanium (Ti) film and a titanium nitride (TiN) film. Then, a tungsten film 4 is deposited by a chemical vapor deposition (CVD), as shown in FIG. 1B. Furthermore, a photoresist pattern 5 for a possible wiring conductor is formed by a lithography, as shown in FIG. 1C. The tungsten film 4 is etched by a dry etching using the photoresist pattern 5 as a mask, so that a desired wiring conductor 6 is completed as shown in FIG. 1D.
Next, another example of the prior art will be described. In a semiconductor device having an upper level metal wiring conductor and a lower level metal wiring conductor which are separated by an interlayer insulator film such as a silicon oxide film but which are interconnected through a contact hole formed by selectively etching the interlayer insulator film, it was a conventional practice of forming the metal wiring layers by a relatively simple and inexpensive sputtering. However, because a diameter of the contact holes has become small with advanced microminiaturization, a void occurs in the contact hole if sputtering having a poor step coverage is used. A long term reliability of the wiring conductor is deteriorated because of the void. Therefore, the metal wiring conductors have been formed by a chemical vapor deposition having an excellent step coverage to prevent generation of the void. However, if the metal wiring conductors are formed by the chemical vapor deposition, it is disadvantageous because a silicon wafer is bowed from internal stress of the metal film. This often results in a suction failure in a next step performed in the same apparatus or in a vacuum suction robot feeding machine for feeding it to a next step.
In order to avoid the above problem, the conventional semiconductor device was formed as follows: As shown in FIG. 2A, an silicon oxide film 12 is formed on a silicon substrate 11 in which a desired circuit has been already formed. Then, a contact hole 13 is formed by selectively etching the silicon oxide film 12, and a refractory metal layer 14 is deposited on a surface including the contact hole 13, by a sputtering.
Thereafter, as shown in FIG. 2B, a tungsten film 15 is formed by a chemical vapor deposition using WF6 and H2 as a raw material, under a condition realizing an excellent step coverage, namely, a reaction rate controlling condition such as a WF6 flow rate of 50 sccm to 100 sccm and a deposition temperature of 400xc2x0 C. to 460xc2x0 C.
Then, as shown in FIG. 2C, the tungsten film 15 is etched back so that the tungsten film 15 remains only in the inside of the contact hole 13 but the tungsten film 15 is completely removed from the upper surface of the refractory metal film 14 so as to expose the surface of the refractory metal film 14.
Furthermore, as shown in FIG. 2D, an aluminum layer 17 is deposited by a sputtering, and then, is patterned to form a wiring conductor composed of the refractory metal film 14 and the aluminum wiring layer 17 which are connected to a lower level circuit in the contact hole 13.
In the first example of the prior art as mentioned above, the tungsten film formed by the blanket WCVD is required to fill a fine contact hole having a diameter of not greater than 0.5 xcexcm without occurrence of the void, and also to have a property which causes no problem in a process after the tungsten film deposition step. In the prior art, however, the tungsten film used for forming a tungsten plug can fill the fine contact hole with no void, but has the internal stress of 1xc3x971010 dyne/cm2 or more, so that the wafer has a large bowing. Therefore, if the wiring conductors were formed of the tungsten film used for forming the tungsten plug, a problem is encountered in which, a feed trouble occurs in a feeding system of a semiconductor manufacturing machine for feeding it to a process after the tungsten film deposition step, particularly, a lithography step for forming the wiring pattern.
In order to lower the stress of the tungsten film, it is a general practice to form a film deposition under a supply rate controlling condition by lowering the WF6 flow rate and/or by elevating the deposition or growth temperature.
As shown in FIGS. 3A and 3B, if the metal film were formed by changing the CVD film deposition condition of a tungsten film 16 to a condition for forming a film having a low internal stress, namely, to a supply rate controlling condition, the step coverage property is deteriorated, so that a void 18 occurs, and a problem such as a low long-term reliability of the wiring conductor and the others is generated.
As mentioned above, with the film deposition under the supply rate controlling condition, the step coverage of the tungsten film deteriorates, and it becomes difficult to fill a fine contact of not greater than 0.5 xcexcm without generation of void.
Because of the problems mentioned above, it was difficult to use the tungsten film for the wiring conductor having small contacts.
In the second example of the prior art as mentioned above of the semiconductor device and the method for manufacturing the same, the manufacturing process becomes long and complicated, resulting in an increased manufacturing cost. In addition, in order to shorten the manufacturing process, if the metal wiring conductor is formed of a metal film formed by only the CVD process, there occurs the above mentioned problem caused by the high stress.
Thomas E. Clark, et al, xe2x80x9cHigh Pressure Blanket CVD Tungstenxe2x80x9d, Solid State Technology Japanese Edition, December 1989, pages 33-41 discloses a different WCVD process. However, the tungsten film formed in this process has a high internal stress, and therefore, cannot be used as the wiring conductor.
Accordingly, it is an object of the present invention to provide a semiconductor device and a method for manufacturing the same, which have overcome the above mentioned defects of the conventional ones.
Another object of the present invention is to provide a semiconductor device and a method for manufacturing the same, which have a tungsten plug and a tungsten wiring conductor, and which have a minimized wafer bowing and an excellent step coverage.
Still another object of the present invention is to provide a method for manufacturing a semiconductor device, which has a reduced number of manufacturing steps.
The above and other objects of the present invention are achieved in accordance with the present invention by a semiconductor device having a semiconductor substrate having a principal surface thereof, an interlayer insulator film formed on the principal surface of the semiconductor substrate, a contact hole formed through the interlayer insulator film to reach the principal surface, and a metal wiring conductor filling the contact hole to reach the principal surface and extending on the interlayer insulator film, the metal wiring conductor being formed of a lower level metal film having an excellent step coverage to fill the contact hole with no void, and an upper level metal film formed on the lower level metal film, the upper level metal film being formed of the same metal as that of the lower level metal film, and the upper level metal film having an internal stress smaller than that of the lower level metal film.
In one embodiment, the lower level metal film having an excellent step coverage has a film thickness not smaller than 40% of a diameter of the contact hole, and the remainder of a desired thickness is constituted of the upper level metal film having the internal stress smaller than that of the lower level metal film.
According to another aspect of the present invention, there is provided a method for forming a semiconductor device having a wiring conductor formed of tungsten, the method including the steps of forming a tungsten film by a low pressure chemical vapor deposition, injecting ions into the tungsten film, and patterning the ion-injected tungsten film so as to form a wiring conductor composed of the ion-injected tungsten film.
Preferably, the above method further includes the step of heat treating the ion-injected tungsten film before the patterning step. Furthermore, an internal stress of the tungsten film is reduced to a value not greater than 8xc3x97109 dyne/cm2 by the ion injection and the heat treatment or only by the ion injection.
According to still another aspect of the present invention, there is provided a method for forming a semiconductor device having a semiconductor substrate a principal surface thereof, an interlayer insulator film formed on the principal surface of the semiconductor substrate, a contact hole formed through the interlayer insulator film to reach the principal surface, and a metal wiring conductor filling the contact hole to connect with the principal surface and extending on the interlayer insulator film, the method including the step of forming a lower level metal film having an excellent step coverage on the interlayer insulator film including an inside of the contact hole, and the step of forming on the lower level metal film an upper level metal film having an internal stress smaller than that of the lower level metal film.
Preferably, the lower level metal film having an excellent step coverage has a film thickness not smaller than 40% of a diameter of the contact hole, and the remainder of a desired thickness is constituted of the upper level metal film having the internal stress smaller than that of the lower level metal film.
In one embodiment, the lower level metal film and the upper level metal film are continuously formed.
In one preferred embodiment, the lower level metal film is formed by a chemical vapor deposition under a reaction rate controlling condition, and the upper level metal film is formed by a chemical vapor deposition under a supply rate controlling condition.
Specifically, the lower level metal film and the upper level metal film are formed of tungsten by a chemical vapor deposition while maintaining a deposition temperature at a constant value within a range of 400xc2x0 C. to 500xc2x0 C., but the lower level metal film is formed by controlling a WF6 flow rate on the order of 50 sccm to 100 sccm and the upper level metal film is formed by controlling a WF6 flow rate on the order of 10 sccm to 50 sccm.
Alternatively, the lower level metal film and the upper level metal film are formed of tungsten by a chemical vapor deposition while maintaining a WF6 flow rate at a constant value within a range of 10 sccm to 100 sccm, but the lower level metal film is formed by controlling a deposition temperature on the order of 400xc2x0 C. to 450xc2x0 C., and the upper level metal film is formed by controlling a deposition temperature on the order of 450xc2x0 C. to 500xc2x0 C.
As still another embodiment, the lower level metal film is formed of tungsten by a chemical vapor deposition by controlling a deposition temperature on the order of 400xc2x0 C. to 450xc2x0 C. and by controlling a WF6 flow rate on the order of 50 sccm to 100 sccm, and the upper level metal film is formed of tungsten by a chemical vapor deposition by controlling a deposition temperature on the order of 450xc2x0 C. to 500xc2x0 C. and by controlling a WF6 flow rate on the order of 10 sccm to 50 sccm.
Moreover, the lower level metal film and the upper level metal film are continuously formed so as to control a stress of the whole of the lower and upper level metal film by changing a film thickness ratio between the lower level metal film and the upper level metal film.